We all wake occasionally during the night. We hear a noise — traffic or the chirp of birds — we fidget, there's a flash of light or a change in temperature.
It is what researchers call "cortical arousal" — a brief period of unconscious wakefulness. And, though such arousals affect our heart rate, blood pressure and blood flow (cardiac haemodynamics), they are normal.
But when those nightly arousals become frequent, they can become a "burden."
Researchers who study sleep (polysomnologists) speak of nocturnal "arousal burden": the burden of being aroused at night.
They have shown that arousal burden can disrupt the natural circadian rhythm of the body's cardiovascular system. And that can lead higher blood pressure and insulin resistance.
Now, a study of 8,001 people, spread across various test groups, suggests that there is a link between arousal burden and cardiovascular and "all-cause" mortality — that is, bad sleep can be fatal.
Cardiovascular diseases (CVDs) are the No. 1 cause of death globally. The World Health Organization says "more people die annually from CVDs than from any other cause." At the last count, it was 17.9 million deaths from CVDs in one year.
Researchers from Australia, the Netherlands, Denmark and the United States have published results from a long-term three-part study in the European Heart Journal.
They say the mortality risks from arousal burden comes down to quality of sleep, rather than how long you sleep.
Watchwords are "sleep irregularity," difficulties getting to sleep and "nonrestorative sleep" — basically, waking up feeling worse than when you went to bed. All that is "associated with increased risk of mortality, irrespective of sleep duration," they write.
The participants were an older group of people, and most of them had preexisting health conditions.
For instance, 40% of the women were overweight, 13.7% had diabetes, and 60% had a history of hypertension.
Half the men had a history of hypertension, 13% had diabetes, and 17% had histories of coronary artery disease.
The researchers acknowledge the age and social backgrounds of the participants as a limitation in their findings: "All cohorts comprise predominately white men and women of predominantly middle to older age. Hence, our findings cannot be extrapolated to other races or younger individuals."
On the other hand, it was perhaps useful for the purpose of studying the effect of underlying conditions.
As the WHO points out: "Most cardiovascular diseases can be prevented by addressing behavioural risk factors such as tobacco use, unhealthy diet and obesity... People with cardiovascular disease or who are at high cardiovascular risk (due to the presence of one or more risk factors such as hypertension, diabetes, hyperlipidaemia or already established disease) need early detection and management using counselling and medicines, as appropriate."
So, if you're not getting good sleep on a regular basis, you may want to consult a doctor, regardless of your age or background. It can't hurt to have yourself checked out by a qualified medical professional.
Death statistics over a decade
The researchers recorded how many of the participants had died and from what.
Comparing the results from the first two parts of the study, it appears that 8.4% men and 11% of women died of some form of cardiovascular condition.
The third part of the study, which involved almost 5,000 people, showed deaths of 987 participants, including 344 cardiovascular deaths, over a period of at least 11 years.
The researchers say that "while the frequency of arousals was lower in women than in men, the association with mortality was stronger in women."
They also say they did not consider the effects of any medication that the participants took, and their findings were obtained on a single night (an overnight polysomnography).
They say that there may have been night-to-night variations in arousal burden among the participants, and that may have influenced the "strength of the associations" with cardiovascular and all-cause mortality.
In future studies, the researchers say they may look deeper into the frequency of disrupted sleep and how participants transition from deep to light sleep, with a view to reduce arousal burden and the associated health risk overall.